Since many facial expressions and the emotions they represent are common to the entire human species, our brains are wired to search for and interpret these facial cues. Likewise, when we feel something on an emotional level, our faces have a peculiar habit of exhibiting an expression that matches the way we feel, sometimes without our knowledge or without the desire to put our emotions on display across our faces.

This pesky trait can be quite revealing, even though an unintended expression may last only a fraction of a second. These superfast expressions that suddenly appear in the middle of another -- sometimes opposite -- expression are microexpressions.

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While we provide others with visual information about the way we feel through our expressions, other information "leaks" out of our faces between or during these intentional expressions. Microexpressions can be as brief as about 1/25 of a second [source: Ekman]. They occur so fast that they're often not perceived by the conscious mind of either the expresser or the person observing the expression. As few as 10 percent of people are even aware of seeing microexpressions when tested [source: Zetter].

Microexpressions can be much more accurate signs of a person's true feelings and intentions than the expression he or she is consciously producing. The smiling salesman may flash a millisecond-long sneer of contempt, or the fierce-looking man approaching you in the parking lot may have a sudden look of fear wash across his face.

Even when we're consciously unaware of detecting a microexpression, it still influences our brain activity and alters our perception of the expressions we do consciously see on another's face. So, if you see a "happy" expression plain as day on someone's face and there were no microexpressions preceding it, you'll identify it as happy. But if the "happy" expression is preceded by a sneering microexpression that you aren't even conscious of detecting, you'll be more likely to describe that same "happy" face as "cunning" or "untrustworthy." This may go a long way in explaining that uneasy feeling that leads you to walk away from someone thinking, "I don't know what it is, but there's just something about that person …"

This subconscious processing can slip into overdrive. Studies have shown people with ­social anxieties have more brain activity than normal when they subconsciously detect a "fear" microexpression. Since threatening situations produce negative feelings for them, detecting signs of fear in other other people creates the same anxious response [source: Tremmel]. Making matters more complicated, the expressions for "fear" and "surprise" are very similar and often confused by observers.

Ekman shares his knowledge with law enforcement and intelligence agencies to help them better detect suspicious behaviors or deceptions, such as a millisecond-long look of fear expressed by someone approaching an airport security check. Ekman believes the ability to detect and interpret microexpressions can be improved by studying changes in human faces using photographs or video.

Spotting terrorists in line at the airport is just one extreme use of the ability to detect microexpressions. It can also benefit us in normal situations. When we misperceive microexpressions, we can make false assumptions about the people with whom we communicate. This creates distance in our relationships, instead of increased awareness. By tuning into microexpressions, we can improve our chances of seeing a big sign from a very fast sign vehicle.

For more articles you might like, like How Lying Works and what facial blindness is, keep clicking to the next page.